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GLOBAL JOURNAL OF ADVANCED ENGINEERING TECHNOLOGIES AND

SCIENCES

EFFECT OF FLY ASH ADDITION ON MECHANICAL PROPERTIES OF

CONCRETE Farhat Abubaker*, Hadria Ghanim and Ashraf H M Abdalkader

* Department of Civil Engineering, Omar Al-Mukhtar University, El-Beida, Libya, Department of

Architecture Engineering, Omar Al-Mukhtar University, El-Beida, Libya, Department of Civil

Engineering, Omar Al-Mukhtar University, El-Beida, Libya

ABSTRACT Although, the use of pulverised fuel ash (PFA) as cement replacement has been widely reported to enhance

concrete durability against chemical attack, no enough information is published addressing the effect of high

volume fly ash replacement on mechanical properties of concrete. Thus, the current experimental study was

carried out to investigate the effect of PFA replacement (10, 25 and 50%) on mechanical properties of concrete.

Performance of PFA concrete specimens was compared to specimens made with CEMI (42.5 N). Two different

water to binder ratios (0.35 and 0.45) were used. Compressive and flexural strength were evaluated at 7, 28, 60

and 90 days of curing. Workability of fresh concrete mixes was assessed using slump test. The carbonation depths

were also tested using 100 mm cube specimens at 90 days of curing. Replacement of cement by fly ash improves

the workability of concrete, but causes a reduction in compressive strength and flexural strength of concrete. The

reduction increases as the replacement by PFA increases. Replacement of cement by 50% PFA showed decrease

in compressive and flexural strength by about 36 and 43 %, respectively compared to CEMI concrete mixes at 90

days.

KEYWORDS: Fly ash, compressive strength; flexural strength, carbonation depth, workability.

INTRODUCTION

The need of high levels of energy by the cement industry causes emission of high levels of carbon dioxide (CO2)

in air. Different supplementary cementitious materials such as fly ash, slag and silica fume are usually used as

cement replacement to reduce the quantity of Portland cement in concrete, which assists in reducing the embodied

energy and associated CO2 emission. Fly ash (pulverized fuel ash) is a by-product of an electricity generating

plant using coal as fuel. Since, a large amount of fly ash is produced every year, its disposal become a major

environmental issue in many countries. Thus, the incorporation of fly ash into concrete has been accepted to be

one way for its disposal [1,2].

It is known that the durability of concrete depends on its physical properties, which might be controlled by

different factors such as water to cement ratio, cement type and additives [2]. The incorporation of fly ash in

concrete causes reduction in water demand of concrete, reducing water/cement ratio for equal consistence and

improve workability of concrete [3,4,5]. For a constant workability, the reduction in the water demand of concrete

due to fly ash is usually between 5 and 15 per cent by comparison with a Portland-cement-only mix having the

same cementitious material content. Furthermore, the reduction is larger at higher water/cement ratios. This is

due to increase the paste volume (its spherical shape and fines) that lead to the increase of plasticity and cohesion.

A concrete mix containing fly ash is cohesive and has a reduced bleeding capacity. The mix can be suitable for

pumping and for slip forming; finishing operations of fly ash concrete are made easier [4,5,6]. Dhir et al. [3] states

that the addition of fly ash improves the dispersion of the Portland cement particles, improving their reactivity. In

addition, the partially replacement of Portland cement by fly ash can also improve the durability of concrete by

controlling the high thermal gradients, pore refinement and resistance to chloride and sulfate in the long-term

performance [7,8]. As fly ash pozzolanically reacts with lime, this potentially reduces the lime available to

maintain the pH within the pore solution. However, fly ash does reduce the permeability of the concrete

dramatically when the concrete is properly designed and cured [2,6,9].

Although, PFA concrete tend to have low compressive strength at early ages, its properties may be improved at

later ages [5,6,10]. This can be attributed to the slow reaction rate of fly ash, producing concrete with relatively

less engineering properties at early ages [6,11]. The refinement of pore structure and the consequence reduction

in permeability make the PFA concrete less susceptible to deterioration [4-15].

According to BS EN 197-1, BS 8500 and BS EN 206- 1, there are two categories of fly ash used in concrete:

regular fly ash and high-volume fly ash. The regular fly ash concrete utilizes about 25–30% of fly ash by weight

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of Portland cement and is widely used for massive structures, such as gravity dams [2]. While BS EN 197- 1

permits the use of fly ash of up to 55%, data from the European Ready Mixed Concrete Organization show that

the cement addition content was less than 20% of the total cement consumption in ready-mixed concrete. [6].

In this experimental study, the effect of different PFA replacement (10, 25 and 50%) on mechanical properties of

concrete were investigated. Performance of PFA concrete specimens was compared to specimens made with

CEMI. Two different water to binder ratios (0.35 and 0.45) were used. Compressive and flexural strength were

evaluated at different curing ages (7, 28, 60 and 90 days). Slump test was performed on fresh concrete mixes. The

carbonation depths were also tested using 100 mm cube specimens at 90 days of curing.

EXPERIMENTAL WORK Materials used

Cement

CEM I (42.5 N), manufactured by Helwan Company-Egypt, conforming to the requirement of BS EN 197-1:2011

[16] was used. The chemical composition of the cement are given in Table 1.

Pulverized -fuel ash (PFA)

The pulverized –fuel ash (PFA) was obtained from Ash Sika Egypt for construction chemicals, and conformed to

BS EN 450-1:2005+A1:2007 [17]. The chemical composition of the cement is presented in Table 1.

Table 1: Chemical composition of CEMI and PFA

Chemical Composition

(%)

CEMI PFA

SiO2 22.19 51

CaO 62.74 2.6

Al2O3 4.08 24.8

Fe2O3 3.16 10.7

Na2O 0.2 1.04

K2O 0.5 3.4

MgO 1.09 1.6

SO3 3.1 0.52

LOI 2.52 -

Fine and coarse aggregate

Natural sand with an apparent specific gravity of 2.63 and absorption of 0.5% was used. Crushed siliceous

aggregate with maximum size (10 mm), specific gravity of 2.67 and absorption of 1.5%, complying with BS EN

12620:2002+A1:2008 [18] were used.

Water

Potable tap water available at the laboratory used in concrete mixtures in this research

Mixture Proportions and mixing procedures Mix design proportioning was performed by using weight-batching method and was designed in accordance with

the Building Research Establishment (British method)[19]. Proportioning of concrete mixtures is shown in Table

2. All mixtures were mixed in a laboratory pan mixer with a capacity of 56 liters. The mix ingredients placed in

the mixer was in the following order; dry aggregates and cement were mixed in the mixer for 30 seconds then

water was added gradually in 15 seconds and the mixing continued for 2 minutes. Therefore, the total mixing time

was 3 minutes for each concrete mixture. After mixing, A series of 100-mm cubes and prisms (100 x 100 x 500

mm) concrete specimens were cast in pre-oiled moulds and fully compacted using vibration table and the top

surface was leveled and finished by trowel.




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Table 2: Proportioning of concrete mixes (kg/m3)

Mix Proportion (kg/ m3)

Mix Coarse

Aggregate Sand Water w/b PFA CEMI

815 1050 133 0.35 0 380 CEMI

850 1145 171 0.45

815 1050 133 0.35 38 342 CEMI-10%PFA

850 1145 171 0.45

815 1050 133 0.35 95 285 CEMI-25%PFA

850 1145 171 0.45

815 1050 133 0.35 190 190 CEMI-50%PFA

850 1145 171 0.45

Curing of test specimens

After casting, the specimens were covered with wet hessian and plastic sheets for the first 24 hours at laboratory

temperature 20±2°C. After 24 hours, specimens were removed from the mould and kept in water curing (Fig. 1)

at 20°C until the test age.

Figure 1: Concrete specimens in curing water

Experiments on fresh concrete

The workability of freshly mixed concrete was measured by using slump test according to BS EN 12350-2:2009

[20] .

Compressive and flexural strength tests

Compressive strength were performed on standard 100 mm cubes according to BS 1881:Part 116:1983[21]. For

flexural strength test, prisms of 100x100x500 mm were used. Each prism was supported on a steel roller bearing

near each end is loaded through similar steel bearings placed at the third points on the top surface according to

BS EN 12390-5. For each test three specimens were tested at 7, 28, 60 and 90 days of water curing.




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Carbonation depth

Carbonation is generally tested by an indirect method using phenolphthalein. A solution of phenolphthalein

indicator is applied onto a fresh cut concrete surface. The indicator changes the colour at pH about 9. The un-

carbonated concrete (pH > 9) shows a violet colour, while the carbonated concrete with a lower pH remain

grey[22].

The carbonation depth measurements were performed on 100 mm cube specimens. The normal method of

determining the depth of carbonation of concrete is to spray concrete surface, or the broken surface itself, with a

solution of the indicator phenolphthalein made up as a 1 % solution in alcohol-water. The specimens were tested

at 90 days.

RESULTS AND DISCUSSION Fresh Properties

The results of slump test are presented in Figure 2. It can be seen from the figure that concrete workability

increased as the replacement by PFA and the w/b ratio increased in all concrete mixes. Concrete mix made with

CEMI and w/b ratio of 0.35 showed the lowest value of slump (35 mm), while the slump increased up to 160 mm

for mix made with 50% PFA. Replacement of cement by 50% PFA caused an increase in slump by about four

and three times compared to CEMI mix when w/b ratio of 0.35 and 0.45 used, respectively. Previous studies

[1,23,24] showed that for a constant workability, the reduction in water demand of concrete due to the addition of

fly ash usually lay between 5 and 15 per cent and the reduction is larger at higher water/cement ratios.

Figure 2: Slump values for all concrete mixes

Compressive Strength

The compressive strength results for concrete made with 0.35 and 0.45 water to binder ratios are shown in Figures

3 and 4, respectively. Generally, increasing the fly ash content in concrete lead to reduction in the strength of

concrete. This is expected as the secondary hydration reaction due to pozzolanic action is slow at initial stage. All

specimens showed increase in compressive strength values with age. The highest value of compressive strength

(48 MPa) was recorded for CEMI concrete specimens at 90 days. The replacement of cement by PFA caused an

obvious reduction in compressive strength values. A value of 28 Mpa was gained for concrete specimens made

with 50 %PFA replacement and 0.45 w/b ratio at 90 days of curing. The results also showed that the compressive

strength of concrete specimens decreased as the water to binder ratio increased. When water to binder ratio was

increased from 0.35 to 0.45, slight differences in compressive strength values for concrete made with CEMI and

10% PFA were shown, while reduction by about 23% and 31% in compressive strength were observed when 25%

and 50% PFA replacement were used.

Dias et al, [6] stated that a mix containing PFA may develop lower strengths at early ages. At later age, the

additional formation of calcium silicate hydrates due to the use of PFA would increase density, refine the pore

3545

60

160

80

100

140

240

0

25

50

75

100

125

150

175

200

225

250

275

CEM I 10 % PFA 25 % PFA 50 % PFA

slu

mp

(m

m)

w/b=0.35

w/b=0.45




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structure of concrete [25,26,27]. Therefore, fly ash concrete with equivalent or lower strength at early age may

have equivalent or higher strength at later age [11,14].

Figure 3: Compressive strength of concrete with w/b of 0.35

Figure 4: Compressive strength of concrete with w/b of 0.45

Flexural Strength

The results of flexure strength are illustrated in Figures 5 and 6. Generally, the flexural strength values showed

slight decrease as the water to binder ration increased from 0.35 to 0.45 at 90 days. On the other hand, obvious

decrease in flexural strength values were noted when the PFA replacement increased. Value of about 6 Mpa was

recorded at 90 days for CEMI concrete compared with 3.8 Mpa for concrete made with 50% PFA.

It can be seen that the flexural strength values increased gradually over the age. In addition, the flexural strength

of concrete made with CEMI showed the highest flexure strength value at 7 days, while replacement by 50%

PFA resulted in biggest reduction in the flexural strength value.

0

5

10

15

20

25

30

35

40

45

50

0 10 20 30 40 50 60 70 80 90 100

Co

mp

ress

ive

str

en

gth

(M

pa)

Age (Days)

CEMI

10 % PFA

25 % PFA

50 % PFA

05

101520253035404550

0 10 20 30 40 50 60 70 80 90 100

Co

mp

ress

ive

str

en

gth

(M

pa)

Age (Days)

CEMI

10 % PFA

25 % PFA

50 % PFA




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Figure 5: Flexural strength of concrete with w/b of 0.35

Figure 6: Flexural strength of concrete with w/b of 0.45

Carbonation Depth The results of carbonation depth of concrete curried for 90 days are shown in Figures 7 and 8. The results revealed

that the carbonation depth increased as the water to binder ratio increased and as the PFA replacement increased.

CEM I concrete showed the lowest carbonation depth (less than 1mm) and no noticeable change in the carbonation

depth values were recorded when the w/b ratio changed from 0.35 to 0.45. Concrete made with 50% PFA showed

the highest carbonation depths. These were 7 and 4 mm for PFA concrete made with 0.45 and 0.35 water to binder

ratio, respectively. While, values of 3 and 2.5 mm were recorded for concrete made with 25% PFA and prepared

with 0.45 and 0.35 water to binder ratios, respectively.

The pH of the pore water in hardened Portland cement paste decreases from 13.5 to a value of about 9 due to the

carbonation and this may increase the corrosion risk of steel reinforcement [2, 28, 29].

0

1

2

3

4

5

6

7

0 10 20 30 40 50 60 70 80 90 100

Fle

xura

l str

en

gth

(M

pa)

Age (Days)

CEMI

10 % PFA

25 % PFA

50 % PFA

0

1

2

3

4

5

6

7

0 10 20 30 40 50 60 70 80 90 100

Fle

xtu

ral s

tre

ngt

h (

Mp

a)

Age (Days)

CEMI

10 % PFA

25 % PFA

50 % PFA




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Figure 7: Carbonation depth of concrete

0.35 0.45 W/B ratio

CE

MI

10

% P

FA

25

% P

FA

50

% P

FA

Figure 8: Carbonation depth of concrete

CONCLUSIONS The following findings can be drawn from the obtained experimental results:

1. High volume replacement by fly ash results in increase the workability of concrete.

2. The replacement of cement by fly ash causes a reduction in compressive strength of concrete.

3. The flexural strength of concrete decreases significantly with the replacement by fly ash.

0

1

2

3

4

5

6

7

8

9

10

CEM I 10% pfa 25% pfa 50% pfa

De

pth

of

carb

on

atio

n m

m

w/b 0.35 w/b 0.45




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4. The compressive and flexural strength of fly ash concrete are improved with curing age.

5. Fly ash concrete shows high susceptibility to carbonation, which may affect concrete durability in long-

term performance.

ACKNOWLEDMENTS The authors would like to thank civil engineering laboratory staff at Omar Al-Mukhtar University for their helps

throughout the experimental part of this study.

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GLOBAL JOURNAL OF ADVANCED ENGINEERING …gjaets.com/Issues PDF/Archive-2018/January-2018/2.pdf · According to BS EN 197-1, BS 8500 and BS EN 206- 1, there are two categories of